One type of conventional oil-free rod/piston assembly comprises an aluminum rod die casting, a preformed polytetrafluoroethylene [“PTFE”] compression ring, an aluminum die cast retaining ring and a metal retaining screw or rivet. The aluminum rod die casting includes a crank bore and a lower bezel connected by an I-section rod. When assembled, the PTFE compression ring is clamped between the retaining ring and the lower bezel to form a piston head. Similar structures are proposed in Droege et al. U.S. Pat. No. 3,961,869 and Droege et al U.S. Pat. No. 3,961,868.
Typically, the rod/piston assembly is fitted into a cylinder such that reciprocation of the piston draws air into the cylinder through an intake port and expels the air from the cylinder through an outlet port under pressure. The compression ring presses against the inner surface of the cylinder to provide a pressure-tight seal about the piston head. Conventional PTFE compression rings comprise annuli sufficiently thin to flex against the inner surfaces of the cylinder so as to enhance the seal between the rod/piston structure and the cylinder.
The use of flexible PTFE in forming compression rings has numerous advantages, including high strength and low friction against the inner surface of the cylinder. Nevertheless, cast or sintered PTFE generally is too stiff and brittle to flex in the manner of the compression ring described above without further processing steps, such as stretching. One desirable feature of the prior art rod/piston assembly is that the PTFE ring can be cast or sintered and then processed before it is combined with the aluminum rod die casting to form the assembly.
One drawback to the prior art rod/piston structure is the relatively large number of pre-formed or pre-cast parts which go into its assembly. The number of pre-formed or pre-cast parts required to assemble the finished structure increases the cost of manufacturing the structure. Likewise, separately fitting the compression ring over the lower bezel of the aluminum rod die casting; fitting the retaining ring over the lower bezel; and fastening the retaining ring to the lower bezel with the retaining screw or rivet adds to the time and cost of assembling the structure.
The number and arrangement of the parts also affects the dimensional repeatability of the structure. That is, the overall length of the rod/piston assembly depends on several factors, including cumulative variations in the lengths of the aluminum rod die casting, the PTFE compression ring and the retaining ring; the elasticity of the compression ring; and the torque or force used in tightening the retaining screw or rivet to clamp the parts together. Controlling all of these factors would be difficult and any steps taken to do so would increase further the cost of the finished assembly.
Dinkel et al. U.S. Pat. No. 6,200,109 proposes an electric motor/pump assembly including a motor shaft and a sealing element which acts upon the motor shaft radially. The sealing element is made of an elastic material, for example, silicone plastics, which is injected into a groove to seal a gap where the sealing lip of the sealing element abuts on the motor shaft side.
Heine et al. U.S. Pat. Nos. 6,139,023; 5,874,170; and 5,756,025, the disclosures of which are incorporated by reference, propose a method for injection molding a seal carrier to a sealing member. The sealing member is modified by admixture or surface treatment with a material compatible with the seal carrier. The modified sealing member then is placed in an injection mold. The mold is closed and plastic material which will form the carrier body is injected into the mold. The sealing member fuses to the plastics of the carrier body as the carrier body cools, thereby providing a durable connection in substance lock.
Ebbing U.S. Pat. No. 5,282,412 proposes an angulating piston assembly for a vehicle air conditioning system compressor. The proposed angulating piston assembly is made up of two components: namely, a composite piston and a piston ring subassembly that includes a rigid annular support collar and a stretched piston ring on the collar. A proposed method for making the angulating piston assembly includes the steps of cutting or skiving an elastomeric ring if desired width from an extruded tube of mineral or graphite filled PTFE, stretching the elastomeric ring, sliding the elastomeric ring onto the support collar to form the piston ring subassembly; placing the piston ring subassembly into one half of a mold cavity for forming the molded head of the composite piston; positioning a distal end of a piston rod so as to extend into the same half of the mold cavity; closing the other half of the mold cavity to fully capture the piston ring and the subassembly; and injecting high temperature, high strength glass-reinforced resin into the cavity in order to form the piston head.
Preston et al. U.S. Pat. No. 4,986,553 proposes a shaft seal comprising a rigid case ring and a PTFE sealing ring component which is secured to a radial flange of the case ring by being bonded to a molded elastomeric sealing ring component.
Various patents, including Kanari et al. U.S. Pat. No. 5,611,260; Holtzberg U.S. Pat. No. 4,432,925; and Dorsch U.S. Pat. No. 4,462,767, propose mechanical techniques for securing PTFE rings to piston heads.
Thus, there remains a need in the art for an improved rod/piston assembly comprising relatively few pre-formed or pre-cast parts and an improved method for manufacturing such an assembly requiring relatively few production steps. In particular, there is a need in the art for an improved method which is particularly, though not exclusively, suited for use in manufacturing rod/piston assemblies comprising pre-formed PTFE compression rings and pre-formed or pre-cast metal rod members having repeatable dimensions.